1. Field of the Invention
The present invention relates to an image processing apparatus for applying tonal processing on input pixel information.
2. Related Background Art
For obtaining a halftone image from a digital printer or the like there have been proposed various methods, including the dither method and density pattern method.
These methods are used in various fields because:
(1) an image with intermediate tone can be reproduced with a binary display device; PA1 (2) they can be easily realized by hardware; and PA1 (3) they can achieve image quality of a certain level. PA1 (1) employing small threshold value matrices as the unit of resolving power; and PA1 (2) employing large matrices as the unit of tonal rendition.
More specifically, as shown in FIGS. 2A and 2B, pixels 8 of an input image (input pixel information) are compared with components of a threshold value matrix 5, and white or black level in an output image on a display screen 6 is determined according to whether the input pixel information is larger or smaller the threshold values. FIG. 2A represents the dither method, in which a pixel 8 of the input image is compared with a component of the threshold value matrix 5. FIG. 2B represents the density pattern method, in which an input pixel 8 is compared with all the components of the threshold value matrix 5. Therefore, in the density pattern method, a pixel of the input image is represented by plural cells on the display screen 6.
The dither method and the density pattern method are essentially the same, except that an input pixel is compared with a component of the threshold value matrix in the former, with all the components thereof in the latter. Naturally there exist intermediate methods, for example a method of comparing an input pixel with plural ones among all the components of the threshold value matrix (for example 2.times.2 i.e., 4 components in FIG. 2B).
Consequently there is no basic difference between these two methods, so that the dither method, density pattern method and intermediate methods will hereinafter be collectively called the dither methods.
In such dither methods, the threshold value matrix can be prepared in various manners. However not much effort has been made in the development of a method for obtaining an output image of high quality in simple manner. In such dither methods, an output image with improved tonal rendition can be obtained without sacrificing the resolving power, by
The present inventors reported, in a report "Halftone Reproduction for Digital Color Printing in Electrophotography" Journal of the Electrophotographic Society, Vol. 25, No. 1, P31 (1986), a method of dividing a dither matrix into four sub-matrices of which diagonally positioned ones have the same threshold values, and employing a multi-level digitization for obtaining a high resolving power and a high tonal rendition. This method was named by the present inventors the IH (Improved Halftone) method.
The present inventors have expanded this IH method to the image containing screen angles, and have avoided the formation of Moire fringe patterns by selecting different screen angles for different colors in forming a color output image.
Though such methods can be suitable for reproducing intermediate tone, a screened original (or an original composed of screen dots) will generate beats with the threshold value matrix, thus forming so-called Moire fringe patterns and deteriorating the image quality.
Also the image quality is often deteriorated even with the aforementioned IH method by the uneven pitch of output dots, resulting from a face inclination of a rotary polygonal mirror or a fluctuation in rotation of a photosensitive drum, often encountered in a scanning optical system of a laser beam printer or the like.
Besides there, have been proposed various methods for tonal processing of input pixel information, for example those disclosed in the U.S. Pat. Nos. 4,812,910, 4,517,605, 4,595,956, 4,507,685, 4,553,173, 4,783,837, 4,752,822, 4,626,923, 4,626,901, 4,814,886 and 4,800,442, but further improvements would be desirable.